The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
Variable valve timing and variable valve lift are two currently available and relied upon approaches for improving internal combustion engine performance and fuel economy. Broadly speaking, they achieve these goals by matching engine performance to then current torque demands. That is, under significant operator torque demands, such as accelerating onto an expressway, valve timing and lift are adjusted to achieve maximum power output and thus performance. Specifically, in a two state valve lift system, a high profile (lift) cam will be utilized which provides maximum valve lift and opening time when maximum engine torque output is required. While cruising at a constant speed, valve timing and lift are adjusted to achieve maximum fuel economy. Again in a two state valve lift system, a low profile (lift) cam will be utilized which provides reduced valve lift and opening time and improved fuel economy
The existence and utilization of these two variables (lift and timing) and the two limits of their adjustment (high and low in the case of lift and advanced and retarded in the case of timing) quickly raises the questions of when, i.e., under what conditions, and how, i.e., with what electronic and mechanical means, the states of these two variables can be adjusted to achieve one or more engineering and performance goals.
Aside from the direct and inherent operational aspects of such devices, there are also consequential aspects. For example, while a system may sense operating conditions that necessitate a change of valve lift or timing, other operating conditions may not be conducive to such a change or may be upset by such a change. The ultimate criteria regarding whether a change can or should be made is generally the overall vehicle performance and whether a driver would experience any anomalous conditions or transients in engine power output. Thus such changes or adjustments should be made without any perceptible diminution or augmentation of engine power.
In practice, achieving a smooth valve lift state or valve timing transition often necessitates temporary adjustments to other engine operating systems and parameters. For example, momentary spark retard may be utilized to smooth the transition but this has its own adverse consequences. Another option, also with adverse consequences, involves briefly unlocking a locked torque converter clutch.